A pile is a heavy beam of timber, concrete, or steel that extends into the earth and serves as a foundation or support for a structure. Piles are divided into two general categories: displacement piles and replacement piles. Displacement piles are members that are driven or vibrated into the ground, thereby displacing the surrounding soil laterally during installation. Replacement piles are placed or constructed within a previously drilled hole, thus replacing the excavated ground.
A micropile is a small diameter (typically less than 300 millimeters) replacement pile. Micropiles are used mainly for foundation support of a structure to resist static and seismic loading conditions. Over the last several years, micropiles have become popular for use in commercial buildings and transportation structures. Micropiles are also used as in-situ reinforcements for slope and excavation stability.
Micropiles withstand axial as well as lateral loads and may be considered as a substitute for conventional piles or as one component in a composite soil/pile mass, depending on the design concept employed. Micropiles are installed by methods that cause minimal disturbance to structure, soil, and the environment. The small size of the machinery required for installing micropiles permits installation of micropiles in locations having limited access and low head room. This advantage permits the micropiles to be installed within existing structures.
To form a typical micropile, a hole is drilled, reinforcing steel is placed into the hole, and the hole is filled with mortar, or "grout". The process of filling the hole with the grout is called "grouting". A construction sequence of a typical micropile 10 is shown in FIGS. 1A-F. Installation begins by drilling a hole 12 and inserting a casing 14 in the hole. The casing 14 shown in FIGS. 1A-F consists of three elongate, hollow, cylindrical casing segments 14a-c attached end-to-end.
Installation of the casing 14 occurs simultaneous with the drilling of the hole. This occurs because the first casing segment 14a induces cutting teeth (not shown, but well known in the art) at its bottom end. To prepare for drilling, the first casing segment 14a attached to a drill rig (not shown, but well known in the art) and is rotated into the ground. In difficult soil conditions, an internal drill rod 18 with a drill bit 16 on a distal end can be advanced with the casing 14 to aid in drilling. The first casing segment 14a extends around the drill rod 18 and abuts against the backside of the drill bit 16.
Once the first casing segment 14a is in place, the drill rig is prepared for drilling. The first casing segment 14a is drilled to a depth that is less than the length of the first casing segment 14a (FIG. 1A).
A second casing segment 14b is attached to the end of the first casing segment 14a by threading an external set of threads in the end of the second casing segment 14b into internal threads on the top end of the first casing segment 14a. Alternatively, the segments of a casing 14 can be attached to one another by a casing coupler (not shown in FIGS. 1A-F, but well known in the art). A casing coupler is a cylindrical, hollow element with internal threads on opposite ends. If the casing coupler is used, both ends of each of the casing segments will have external threads. The external threads on the top end of the first casing segment are threaded into one end of the casing coupler, and the external threads of an adjacent casing segment are threaded into the opposite end of the casing coupler.
After the second casing segment 14b is attached to the first casing segment 14a, drilling continues until the top edge of the second casing segment 14b is adjacent to the ground. A third casing segment 14c is attached to the end of the second casing segment 14b. This process is continued until the casing 14 extends completely through the upper, looser portions of the soil base (called the "less competent stratum" and designated generally by the numeral 20 in FIGS. 1A-F), and into the solid under-soil (called the "bearing stratum" and designated generally by the numeral 22 in FIGS. 1A-F) (FIG. 1B). Any number of casing may be used to reach the required depth. However, for simplicity, only three casing segments 14a-c are shown in FIGS. 1A-F.
After the casing 14 is in place, the drill rod 18 and drill bit 16 are pulled out of the casing 14 (FIG. 1C). Reinforcements 24, such as steel rebar, are placed down the length of the inside of the casing. The reinforcements 24 can occupy as much as one half the internal volume of the casing 14. After the reinforcements 24 are placed in the casing 14, grout 26 is introduced into the casing by tremie (not shown, but well known in the art) (FIG. 1D).
After the casing 14 is filled with grout 26, the casing 14 is backed out of the drilled hole 12. Further grout 26 is added under pressure to the casing 14 while the casing is being withdrawn so that the hole 12 left by the casing 14 is filled with grout 26 (FIG. 1E). The pressurized grouting and withdrawal of the casing continues until the bottom edge of the casing is adjacent to the top edge of the embedment length in the bearing stratum 22. Casing segments are removed as the casing 14 is withdrawn from the hole 12. In the sequence shown in FIGS. 1A-F, only the third casing segment 14c is detached from the casing 14, and the top end of the second casing segment 14b extends out of the ground after grouting is complete. Preferably, the pressure used during the grouting process is adequate so that the grout 26 is pressed against the inner surface of the hole 12 so as to create a consistent grout/ground bond. The remaining portion of the casing 14 is left in place through the less competent stratum 20 after the pressurized grouting. After grouting, the casing 14 is typically reinserted a set distance into the top portion of the pressure grouted length, allowing a structural transition between the upper encased and lower uncased portions of the pile.
Finally, steel plates 28 (FIG. 1F) are welded to the top of the casing 14. In the casing 14 shown in FIGS. 1A-F, the steel plate 28 is welded to the top of the second casing segment 14b. A concrete footing 30 is cast around the steel plate 28 and the top end of the casing 14. The micropile 10 is now complete.
The structural capacity of the micropile 10 depends largely on the strength of the elements used as the reinforcements 24 and the casing 14. The reinforcements 24 and the casing 14 are typically formed of high transition strength steel, and are designed to resist most or all of the applied load on the micropile 10.
The reinforcements 24 transfer the load applied to the micropile 10 through the grout to the bearing stratum 22. An effective transfer of the applied load can only occur if the micropile 10 is sufficiently anchored in the concrete footing 30 and the bearing stratum 22. The drilling and grouting methods used in the micropile 10 installation allow high grout/ground bond values to be generated along the grout/bearing stratum interface, and properly anchor the micropile in the bearing stratum 22.
Anchoring of the reinforcement 24 and the casing 14 to the concrete footing 30 is provided primarily by the steel plates 28. Thus, the welded connection between the casing 14 and the steel plates 28 serves a vital function for the anchoring of the casing in the concrete footing 30. It has been found that welding of the steel plates 28 to the top end of the casing 14 decreases the ductility of the high-capacity steel in the casing 14 in the areas of the casing affected by the heat of the weld. This less ductile, heat-affected steel can cause a premature failure of the casing steel at the attachment to the steel plates 28. There exists a need for a better structure for anchoring a high strength steel casing to a concrete footing.
During a seismic event (earthquake), lateral movement of the footing 30 can induce a curvature in the portion of the pile 10 below the footing in the less competent stratum 20. This curvature creates a bending moment and stresses in the pile casing, which are greatest in the length of the casing just below the footing. Lateral displacements which induce bending can also occur in applications where the micropile is used as a component of an earth stabilization system. In these applications, the bending moment is greatest at the slide plane of the micropile. There exists a need for a structure that can reinforce the casing threaded joint where the casing is subject to larger bending stresses.